I. Field of the Invention
This invention relates generally to voltage regulator circuits capable of providing a relatively constant DC output voltage independent (within limits) of load and supply voltage fluctuations. More specifically, the invention relates to a circuit for protecting such voltage regulators and associated electronic systems from damage by high-voltage transients in the unregulated supply voltage.
II. Description of the Prior Art
Early electronic systems used bulky, high-power regulators made from many discrete components to regulate a line which supplied all areas of an electronic system. Unfortunately, the impedance of this line and associated connectors caused voltage drops which varied throughout the system. Also, any common impedance in this line between critical parts of the system could allow unwanted coupling.
More recently, the trend in systems design has been toward the strategic placement of monolithic three-terminal voltage regulators to achieve accurate local regulation throughout the system. In typical embodiments of these monolithic voltage regulators, a stable reference voltage is developed on chip and compared with the regulator output voltage by an error amplifier. This amplifier requires both high gain and wide band width to ensure good regulation characteristics and fast transient response. It must also have low temperature drift to maintain a high order of output voltage stability under changing temperature conditions. The error amplifier drives an output stage, sometimes including a pair of transistors in Darlington configuration to achieve a higher current capability. Also located on the chip are the necessary bias supplies and a variety of protection circuits. The prior-art protection circuits can provide for thermal shutdown, output current limiting and safe area operation. A wide variety of monolithic voltage regulators are commercially available from many manufacturers. A thorough understanding of the construction and application of these prior-art voltage regulators can be found in numerous texts, articles, U.S. patents and commercial product application literature of which the Fairchild Semiconductor Voltage Regulator Applications Handbook and the National Semiconductor Voltage Regulator Handbook are examples.
The commercially available monolithic voltage regulators are suitable for use over a wide range of applications. However, one potentially large application for voltage regulators remains only partially satisfied. This application is in the growing market for voltage regulation in an automotive environment. In a typical automotive electronic system, high-voltage positive transients frequently occur on the system input power lines. One well-known large positive voltage transient, commonly referred to as the "load dump transient," occurs when the battery terminal is temporarily disconnected while the alternator is supplying battery charging current. Typical automotive system specifications require the ability to survive worst-case load dump transients ranging from 80 volts to 120 volts. These transients can be destructive to integrated circuit electronics in view of their associated energy content. Although other automotive transients reach peak voltages in excess of .+-.200 volts, their energy content is considerably less than the load dump transient and presents less of a threat to integrated circuit electronics. However, the general characteristics of automotive transients, i.e., large peak voltages, fast rise times and high energy content, are not adequately provided for by protection circuits used in monolithic voltage regulators known in the prior art. A more detailed discussion of automotive transients and their impact on solid-state electronics can be found in an article by William F. Davis on page 419 of the IEEE Journal of Solid-State Circuits, Volume SC-8, No. 6, December 1973, entitled "Bipolar Design Considerations for the Automotive Environment."